Previously, in electrophotographic devices such as a plain paper copier (PPC), a laser printer, a LED printer and a liquid crystal printer, a process has been frequently used in which an image forming process comprising electrification, exposure and development is applied to photoreceptors of the rotary drum type to form toner images, which are transferred to transfer members, followed by fixing, thus obtaining duplicated copies. As the photoreceptors used in these devices, inorganic photoreceptors such as selenium, arsenic-selenium, cadmium sulfide, zinc oxide and a-Si photoreceptors are employed, but organic photoreceptors (OPCs) inexpensive and excellent in productivity and waste disposal are also actively studied and developed. In particular, so-called function separation type photoreceptors in which charge generating layers are laminated with charge transporting layers are excellent in electrophotographic characteristics such as sensitivity, charge property and repetition stability thereof, so that various function separation type photoreceptors have been proposed and came in practice.
As units for charging these photoreceptors, corona charging units are generally widely used which comprises shield plates and thin wire electrodes such as gold-plated tungsten wires as main constituent members. However, these corona charging units have the problems that the devices themselves are large in size and high in cost, and produce a large amount of ozone, which causes generation of discharge products, resulting in image defects and unfavorable environmental circumstances. Then, recently, instead of these corona charging units having many problems, contact charging processes have been variously proposed in which surfaces of photoreceptors are brought into abutting contact with voltage-applied conductive members, thereby directly injecting charge into the surfaces of the photoreceptors to obtain a desired charge potential [JP-A-63-149669 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), etc.].
However, when these contact charging processes are applied to the conventional function separation type organic photoreceptors, repeated use of charging members in direct contact with the uppermost surface layers of the photoreceptors generally significantly wears away the uppermost surface layers to induce a reduction in charge property and changes in sensitivity. As a result, the problem is encountered that the life of the photoreceptors are extremely shortened, compared with the case of using the corona charging system. In particular, when a charge transporting layer in which a low-molecular charge transporting material is molecularly dispersed in a high-molecular binder resin is used as the uppermost surface layer of the photoreceptor, this effect is significant.
As to the wear of these photoreceptive layers, various causes are considered. However, in contact charging, direct charge locally flows in the charge transporting layer in which the low-molecular charge transporting material is dispersed in the binder resin. The stress is therefore applied not only to the surface of the photoreceptor, but also to the inside thereof. In a system in which a direct current (DC) voltage is used together with an alternating current (AC) voltage, the deterioration of the charge transporting material and the binder resin is promoted to a further deeper position. Further, locally ununiform dispersion of the charge transporting material also makes the deterioration thereof ununiform, so that the film strength of the photoreceptive layer is lowered, thus conceivably increasing the wear.
Further, the wear of these photoreceptive layers depends on the height and the frequency of the voltage in which the alternating current is superimposed on the direct current, particularly the alternating current voltage, and the time for which it is applied. The wear amount increases with increases in these values.
FIG. 3 is a timing chart at the time when the direct current voltage and the alternating current voltage are applied by superimposition to a contact charging unit to form images, in a conventional image forming device, wherein the thick line means the switch-on state. As is shown in FIG. 3, in the conventional image forming device, the superimposed voltage of the direct current voltage and the alternating current voltage is continuously applied to the conductive member of the charging unit through each image forming cycle. As a result, the stress is always applied to the surface of the photoreceptor, during operation of the image forming device.